POLAR FRONT
THEORY: published shortly after World War I, in Bergen,
Norway by Vilhelm Bjerknes, his son Jacob, Halvor Solberg and Tor
Bergeron.

Figure 13.1.a:
begins with stationary polar front; high pressure north and south
of the pressure trough (i.e., stationary front) sets up cyclonic
wind shear.

Figure 13.1.b: a
wavelike "kink" forms on the front, in response to an
approaching short wave; the low that forms on the front is known as
a "frontal wave" or "incipient cyclone

Figure 13.1.c: in
about 12-24 hours, a fully developed "open wave"
forms; low now has several closed isobars around its center; precip
develops in advance of the warm front and in a narrow band along the
cold front; a "warm sector" develops between the
two fronts

Figure 13.1.d: the
low "deepens" (i.e., central pressure drops); the
wind circulation increases, as more and more closed isobars encircle
the low center; the faster moving cold front begins to overtake
(i.e., "occlude") the slower moving warm front; clouds and
precip cover a large area

Figure 13.1.e: the
system reaches its most intense point and it begins to
"occlude" as the cold front overtakes the warm
front; the low begins to "fill" (i.e., central
pressure rises) and the dissipation stage begins; clouds and precip
in "comma" shape; a secondary low may develop on the
"triple point"
(i.e., where all three fronts meet) and go through a "life
history" of its own

Figure 13.1.f: the
system dies out, as it is far removed from the "warm sector"
and the supply of energy provided by the rising warm, moist air;
this entire "life cycle" can last from a few days to over
a week

CYCLOGENESIS:
the development or strengthening of a mid-latitude cyclone

EXPLOSIVE or BOMB
CYCLOGENESIS: defined to be a
pressure drop of 24 mb in 24 hours, normalized at 60 deg N Latitude
(Bergen, Norway); translates to about 19 mb in 24 hours at 40 N.

PLANETARY
WAVES ("Rossby Waves"): longwave
troughs and ridges that encircle the hemisphere; anywhere from three
(3) to six (6) longwaves at any time around the hemisphere;
wavelength (trough-to-trough or ridge-to-ridge) in the thousands of
kilometers; tend to be stationary or move eastward at
less than 4 degrees Longitude per day (i.e., about 8 knots) or
move westward (i.e., retrograde); Figure 13.6

SHORT
WAVES: smaller disturbances or ripples imbedded in the
longwaves; tend to move eastward at a speed proportional to the
average wind flow at 700 mb; when short wave drops into longwave
trough, constructive interference causes trough to deepen; Figure
13.7a & b

BAROTROPIC:
isotherms are parallel to height contours; air
density does not vary

BAROCLINIC
INSTABILITY: occurs in
a flow pattern when warmer air rises and colder air sinks;
this sets up areas of convergence and divergence,
which in turn intensifies the corresponding surface high and
low pressure areas;Figure
13.8b

The sinking of cold air and the rising of warm air provides
ENERGY to the developing cyclone, as potential energy
is converted to kinetic energy; furthermore, if clouds form,
condensation in the ascending air releases latent heat,
which warms the air; the warmer air lowers the surface
pressure; the cyclone intensifies;
Figure 13.11

The overall effect of
differential temperature advection is to intensify the
wave.

CAA into a
trough, "deepens" the trough; WAA into a
ridge, "builds" the ridge

CONVEYOR
BELT MODEL:
a 3-D model of a developing wave cyclone

Warm Conveyor Belt: the warm conveyor
belt originates at the surface in the warm sector (mT air mass) and
flows northward, slowly rising along the sloping warm front,
gradually turning toward the northeast or east, parallel to the
upper wind flow; the water vapor in the rising air condenses,
forming clouds and precip; Figure 13.12

Cold Conveyor Belt: the cold conveyor
belt originates at the surface to the northeast of the surface low
and north of the warm front (mP); it moves slowly westward from off
the ocean; as it moves into the vicinity of the surface low, rising
air gradually forces the cold conveyor belt upward, turning as it
ascends, to form the comma-shaped cloud pattern; part of the
airstream may rise high enough to get caught in the southwesterly
flow aloft and swings northeastward, thus splitting the cold
conveyor belt; Figure 13.12 and 13.13

Dry Conveyor Belt: this
upper-level airstream slowly descends from the northwest behind the
surface cold front, where it brings general clearing weather; if a
branch of the dry air sweeps into the storm, it produces a clear air
that "sharpens" the comma-head, creating a "dry
slot"; Figure 13.12

Blizzard of 1993: known as
"The Storm of the Century"; the central pressure
dropped to a minimum of 960 mb (28.35"), a pressure comparable
to a Category 3 hurricane, but the surface winds were more like a
Category 1 hurricane in places; the storm produced 27 tornadoes
(mostly in FL, where a "storm surge" was noted as the low
moved ashore from off the NE-Gulf of Mexico); the storm produced
deep snow from AL & GA, north to eastern Canada; every major
airport along east coast was shut down; over 3 million people lost
power; an estimated $800 million in damage and 270 storm-related
deaths; Figures 13.13, 13.14, 13.15 & 13.16